Move Mail to New Server

Hello,
I currently have one server which hosts all services for my ispconfig installation.
I now have added a new dedicated mail server to my installation. I want to move all my domains (about 7) and their respective emails to the new mail server. What is the best way to accomplish this? I want to make sure that this changes also reflect on the control panel.
The new server has already been added to the Master server and I am able to create new mailboxes on it.

I would copy the files from /var/vmail to the new server, change the server_id for mail in the master-db and run the resync-tool afterwards. You may need to add several mail-related db-entries to your new server since the current resync-tool does not sync all mail-settings.